Tales From The Helpdesk

Domain Name System (DNS)

A fundamental part of the Internet, essentially a specialised distributed database, the DNS translates one type of thing into another. The usual type of translation, an “A” (for address) lookup, translates a domain name (the human-readable hostname, e.g. example.com) into an IP address. The address can then be used for sending IP packets to that system.Lookup types:

Lookup type

Usage

A

Address

normal lookup

MX

Mail Exchanger

system accepting mail for the domain

CNAME

Canonical name

PTR

Pointer

reverse lookup

TXT

uninterpreted text

available for any use

SOA

Start Of Authority

HINFO

Host Information

NS

Name Server

AAAA

IPv6 lookup

A6

Alternate IPv6 lookup

Reverse lookups (rDNS)

The special-purpose domain in-addr.arpa can to used to perform reverse lookups, from IP address to domain name. To do this, reverse the groups of the address, append .in-addr.arpa and do a PTR lookup. E.g. to discover a name for 217.146.107.7 do a PTR lookup on 7.107.146.217.in-addr.arpa.

Tools

On unix systems, “dig” and “nslookup”.

Resolver software

The most popular on Unix systems is “bind”.

DNSBL (DNS-accessible blacklists)

These use the capability of the DNS to publish lookup capability into lists of IP addresses. Typically the query is done as a “A” lookup of the reversed IP prepended to the list name. Some also offer TXT records.